Depends on how you want to define “Buckshot”. The common (and some not so common) Buckshot sizes run from T (.20”) to 0000 (.375”). Of them, the largest I’ve seen/loaded/fired is 0000, although the largest I’ve worked up a decent load for is 000. The 0000 won’t stack right in a 12 Gauge.
Now if you want to expand the definition a bit to something like “Multiple pellets larger than BBB”, then I’ve loaded some considerably larger.
Three .60” Balls (12 Gauge, 3”)
Three .575” Balls (12 Gauge, 3”)
Two .60” Balls (12 Gauge, 2 ¾”)
Two .575” Balls (12 Gauge, 2 ¾”)
Three .527” Balls (12 Gauge, 2 ¾”)
DC